1. Field of the Invention
This invention relates to electron beam lithography, and more particularly for a system used to detect the location of alignment marks on a substrate so that proper alignment is maintained with an electron beam used to develop a circuit pattern on the substrate.
2. Description of Related Art
Electron beam lithography involves the scanning of a modulated electron beam over a semiconductor wafer to build up a series of circuit patterns on successive layers. Many layers of patterns are normally written; as each new layer is written it must be aligned very precisely with previously written layers. This is accomplished by positioning a martrix of alignment marks on the wafer, scanning the marks with the electron beam, and analyzing the resulting backscattered electron signal as it is affected by the topography of the mark or by the mark material to determine the exact mark locations. The marks are usually in the form of a cross or an "L", and a number of scans are made in each direction across its arms. The backscattered electrons are sensed, typically with a scintillation material and photodetector or an electron detecting diode, and the signals are processed to correlate the pattern of electrons received with a reference pattern. The detected mark locations are then used to compensate for positioning errors caused by the system's limited ability to consistently position the wafer in its holder mechanism, and by topological distortions introduced in the wafer during wafer processing.
A software system is presently available which locates alignment marks by correlating a scanning signal representing the backscattered electrons with a function which represents the opposed edges of an alignment mark. The resulting correlation has positive and negative peaks, one for each edge of the mark. From these peaks the approximate midpoint of the mark can be determined. The accuracy of this approach is improved as successive scans are performed for the same alignment mark to obtain successive sets of positive and negative peaks, one set for each scan.
While this software approach has been found to be highly accurate, it is relatively slow. This is an important limitation, because the time necessary to perform alignment is a significant limiting factor in the throughput of an electron beam lithography system. There is a need for another approach which retains the high accuracy of the software system, but improves its speed so that a greater throughput, and hence more efficient production rate of integrated circuits, can be achieved.